Which condition is typically not acceptable in turbine blades?

In turbine blades, the presence of cracks is a critically unacceptable condition due to the potential for catastrophic failure during operation. Turbine blades experience extreme stresses, and even a minor crack can lead to a significant compromise of structural integrity. This could result in blade failure, which poses a serious safety risk not only to the aircraft but also to its passengers and crew.

In contrast, while discoloration, surface oxidation, and minor chafing might indicate some wear or age-related effects, these conditions are often considered tolerable to various degrees, provided they have not reached a level where they adversely affect the blade's performance or safety. For instance, discoloration could simply be a sign of heat exposure but might not necessarily affect the functionality unless pronounced. Surface oxidation may require monitoring to ensure it does not progress, but it does not immediately warrant rejection of the blade. Minor chafing may require repair or further evaluation but typically does not reach the severity that cracks do.

Thus, cracks represent a significant and immediate concern in turbine blades, leading to their classification as an unacceptable condition in this context.